Introduce nvim namespace: Fix define guards.

Change define guards from NEOVIM_XXX_H to NVIM_XXX_H:
- Change header files.
- Change clint correct guard name calculation.

src/nvim/misc2.h

@@ -1,5 +1,5 @@
-#ifndef NEOVIM_MISC2_H
-#define NEOVIM_MISC2_H
+#ifndef NVIM_MISC2_H
+#define NVIM_MISC2_H
 
 #include "nvim/func_attr.h"
 #include "nvim/os/shell.h"
@@ -48,4 +48,4 @@ int put_bytes(FILE *fd, long_u nr, int len);
 void put_time(FILE *fd, time_t the_time);
 int has_non_ascii(char_u *s);
 
-#endif /* NEOVIM_MISC2_H */
+#endif /* NVIM_MISC2_H */